80% is a pretty common rule of thumb as the minimum you'd want back from a Bonus bet. It's not based on anything as far as I'm aware other than being pretty achievable, round number. Just had never heard 84% before that's all so was curious.Where does 80% come from? $1.86 lines?
42% is being cautious and comes from $1.89 lines (89%) minus the 5% commission (84%). Equals $84 return for a $100 BB.
You should be aiming for higher than that which can often be achieved when backing long priced sports fixed odds with only 5% such as first goal scorer or anytime try scorer bets. Quite often you will find a fixed prices equal to or better than the Betfair lay price with these exotic sports market, so your return on a $50 BB is going to be closer to $50 if you can be bothered searching.
And it also depends on the bookie as well. Some just have horrible odds and markets so you have to accept a bit less, whereas the bigger places you should be looking at over 90% pretty easily if you know where to look. Batsman scoring over 50 was a great one in T20's until Topsport stopped offering the 'not to score 50' option. Would get a 95%+ option almost every match.